1. Field of the Invention
The present invention relates to a method for use in the manufacturing of a fluid dynamic pressure bearing, and more particularly to a method of filling oil into a bearing gap.
2. Description of Related Art
Various fluid dynamic pressure bearings have been proposed for high rotational precision motors. Examples of such motors include spindle motors of recording disk drives, and motors used to drive polygon mirrors in laser beam printers. Generally, fluid dynamic pressure bearings utilize fluid pressure of lubricating fluid such as oil or the like interposed between a shaft and a sleeve which are rotatable relative to one another.
FIG. 1 shows one example of a motor using such a fluid dynamic pressure bearing. A motor using the conventional fluid dynamic pressure bearing comprises a pair of radial bearing sections 4, 4, formed so as to be spaced apart from each other in the axial direction, between an outer peripheral surface of a shaft 2 that is integrally formed with a rotor 1 and an inner peripheral surface of a sleeve 3 in which the shaft 2 is rotatably inserted. Further, a pair of thrust bearing sections 7, 7 are disposed respectively between an upper surface of a disk-like thrust plate 5 that projects from the outer peripheral surface of one edge section of the shaft 1 in the outward direction of the radius direction and a flat surface of a step portion formed on the sleeve 2, and between the lower surface of the thrust plate 5 and a thrust bush 6 that closes one opening of the sleeve 2.
A bearing gap that is a series of minute gaps is formed between the shaft 2 and the thrust plate 5 and between the sleeve 3 and the thrust bush 6. Oil 9 serving as lubricating fluid is continuously retained in the bearing gap without a break.
Herringbone grooves 41, 41 and 71, 71 formed by joining a pair of spiral grooves are formed at the radial bearing sections 4, 4 and the thrust bearing sections 7, 7, whereby maximum dynamic pressure is produced according to the rotation of the rotor 1 at the central section of the bearing section where a joint section of the spiral groove is positioned, thereby counteracting a load on the rotor 1.
The motor described above has a taper seal section 8 in the vicinity of the upper edge section of the sleeve 3 that is positioned opposite to the thrust bearing sections 7, 7 in the axial direction, so that the surface tension of the oil and the atmospheric pressure are balanced to form an interface. Specifically, the internal pressure of the oil in this taper seal section 8 is maintained at a pressure substantially equal to the atmospheric pressure.
The following method has been proposed for filling the oil 9 retained between the thrust plate 5 and the shaft 2 and between the sleeve 3 and the thrust bush 6 of the bearing section having the above-mentioned construction. Firstly, a vacuum chamber having the oil stored therein is pressure-reduced, and then, with this state, a stirring machine in the oil is operated to perform a stirring and degassing. After the pressure in the vacuum chamber that supports the bearing is reduced to a vacuum level, the oil is supplied to the bearing-supporting vacuum chamber, and a suitable amount of oil is placed at the bearing opening such as the taper seal section 8 or the like of the bearing section under a reduced pressure Subsequently, the environment in the bearing-supporting vacuum chamber is returned to the atmospheric pressure, thereby filling the oil in the bearing gap of the fluid dynamic pressure bearing by utilizing the atmospheric pressure.
However, even in the oil filling method as described above, the oil often bubbles during the filling process. This is because it is extremely difficult, particularly in a mass production process in a factory, to remove the dissolved air to a degree of not forming air bubbles even by stirring and degassing the oil under the reduced pressure. The oil bubbling during the oil filling process hinders a smooth supply from the vacuum chamber having the oil stored therein to the bearing-supporting vacuum chamber. Further, when bubbling occurs at the stage where the oil reaches the bearing-supporting vacuum chamber, the oil may be scattered in a spraying manner in the oil vacuum chamber, thereby staining the bearing and the inside of the chamber with the oil.
The degassing level of the oil is somewhat enhanced by exposing the oil under the reduced pressure environment and performing stirring and degassing. However, effective degassing cannot be carried out by degassing under a state where the oil is stored in the vacuum chamber, since the area exposed to the reduced pressure environment to the volume of the oil, i.e., the surface area of the oil is limited. In this case, it is possible to increase the area to the volume of the oil by using a large-sized vacuum chamber, or by decreasing the amount of oil stored in the chamber. However, these are not realistic solutions since they deteriorate productivity by increasing the size of the oil filling device or by increasing an oil replenishing frequency.